First Admin account not showing on login page

I created a second Admin account on my Macbook Pro (Yosemite) and also enabled Guest login. Now I can only access the second account and the guest login from the login screen, the primary admin account is no longer shown.

It is also not shown in the fast user switching menu, nor am I able to log into it when I select username and password as login option.

I have not activated FileVault and I have not hidden the user by adding it to the HiddenUsersList. Any other things I might check?

I am able to su to the old user and the home directory and all still exist.

What I did (and what seems to have worked so far) is the following:

• change username and home folder of the "missing user"
• create a new user with the old username/home folder
• delete the old user
• use the new user instead

I think I should also have reused the old UID because I got a lot of files belonging to an unnamed "501" user that I had to chown to my new user.